It is desirable in some applications to utilize infra-red emitting electroluminescent (EL) lamp structures for providing illumination of an area in the near infra-red (NIR) region of the spectrum, for example, while minimizing illumination in the visible region of the spectrum. For example, such area illumination can be used on aircraft as formation lights for night vision recognition wherein the lights are designed to avoid as well as possible illumination in the visible portion of the spectrum while providing IR illumination which can be seen by the use of suitable IR sensitive or IR responsive means, such as IR sensors or specially designed IR responsive goggles.
Although an exact line of demarcation is not clearly defined, as used herein, the near infra-red region is generally referred to as lying above about 700 nm. with energy below 700 nm. being generally referred to as lying within the visible range.
Attempts have been made to provide such illumination using known IR emission sources, such as are available using light emitting diodes designed to emit infra-red waves. However, such devices act as point light sources, rather than as area light sources, and it is difficult, for example, for persons responding to such devices to obtain a sufficiently good perception of depth from a single, or even a discretely positioned group, of such point sources. Accordingly, the use of such point source IR emitters has prove generally unsatisfactory in such applications.
Generally the most effective area source of illumination for such applications has been found to be electroluminescent lamp sources which can be designed to provide a relatively uniform illumination over a relatively large spatial region and which can be formed in a variety of shapes and configurations for applications on aircraft, for example, or at other locations.
However, the major spectral energy distribution of electromagnetic wave energy obtained from electroluminescent lamp sources available at the present time is generally concentrated in the visible portion of the spectrum, quite often in the 450-650 nanometer (nm.) range of wavelengths, such as standard white, blue-white, pink-white, aviation green, blue-green, yellow-orange or yellow-green EL lamps, which may have some, but very low, residual levels of emitting energy beyond such range, i.e., above about 650 nm. (normally less than 5% of the total energy emitted being within the range from about 650 nm. to about 1000 nm., or so). Certain EL lamps have been designed to provide what has been termed aviation red illumination and, while such lamps emit energy above 600-650 nm., a reasonable amount of energy below the 600-650 nm. range is also present therein.
Examples of such devices and their spectral energy distribution and chromaticity characteristics are shown, for example, in "Design Guide to Electroluminescent Lighting" (3rd Printing, 1986), published by Loctite Luminescent Systems of Lebanon, N.H. and available from such company. Thus, in order to provide useful emissions within the actuation levels of IR night vision sensors, it is desirable to increase the energy emitted above about 650 nm., or so, relative to the energy emitted below about 650 nm., or so, by as much as an order of magnitude, if possible.